Shatapatha Brahmana 11.1.5.1

11.1.5.1

adyāmāvāsyeti manyamāna upavasati athaiṣa paścāddadṛśe sa haiṣa divyaḥ śvā sa yajamānasya paśūnabhyavekṣate tadapaśavyaṃ syādaprāyaścittikṛta etasmādu haitadbhīṣāvacandramasāditi

He observes the fast thinking, 'To-day is the day of new moon;' and then that (moon) is seen in the west. But, indeed, he (the moon) is that heavenly dog: he watches the Sacrificer's cattle (to seize them), and that would not be good for cattle if amends were not made to them; and through fear of that 'down-coming moon,' as they think him to be,--